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Another Day, Another Deadly Tornado Strikes the US

Published: May 23rd, 2011, Last Updated: May 27th, 2011

Following April's record-smashing tornadoes, which were responsible for searing the devastated city of Tuscaloosa, Alabama into Americans' consciousness, May has brought yet more examples of Mother Nature's capricious power. Yesterday, a massive, violent EF4 tornado temporarily wiped parts of Joplin, Missouri off the map, killing at least 130 people and injuring many more. The Joplin tornado now ranks as the deadliest tornado since 1950.

The fact that this spring has featured such a wide range of devastating weather events, from the 600 or so tornadoes in April to the widespread — and ongoing — Mississippi River flooding — has left me wondering whether the hoopla over the supposed rapture last weekend was not so far off after all. This really is an unusual period in American history, with several major natural disasters unfolding at the same time.

Is this because of La Niña, that sneaky climate-influencing mechanism in the tropical Pacific Ocean? Is it because of global warming? Or are these events simply weather and chance — just a string of horrendous luck?

We don't know for sure, but climate science and meteorological research offer many important clues as to what's behind the different phenomena we've been struggling to cope with during the past several months.

First off, let's look at the La Niña event, which refers to cooler than average water temperatures in the tropical Pacific Ocean. A strong La Niña was in place for much of the winter into early spring (but has since waned), and this helped throw off weather patterns worldwide. Of the recent extreme weather and climate events, La Niña can most directly be linked to the southern drought, and to a lesser extent, the tornado outbreaks.

Research shows that La Niña years favor drier than average conditions in Texas and New Mexico, for example. According to the National Weather Service, Texas just had its driest seven-month period on record, and a greater proportion of the state is experiencing "extreme" or "exceptional" drought conditions than at any time since 2000 (when such monitoring began). Statewide, Texas needs an average of about eight inches of rain in order to significantly ease the drought conditions, with up to 20 inches needed in typically wetter parts of the state and in Louisiana.

Speaking about the Texas and New Mexico drought, which could cost upwards of $1 billion in agricultural and property losses, Victor Murphy of the National Weather Service pinned part of the blame on La Niña. "Basically the La Niña would be far and away the most prominent player, or bad actor, if you will," he told reporters on a conference call today. Murphy is the climate services program manager for the Weather Service's Southern Region.

Graphic illustrating some of the large-scale factors that may have led to the record tornado outbreaks. Credit: Ilissa Ocko for Climate Central.

Climate research has revealed that the larger climate cycle La Niña is a part of — known as the El Niño-Southern Oscillation, or ENSO — may be shifting its behavior in response to warming sea and air temperatures. That is also an area of active climate research, which will be the subject of future coverage here at Climate Central.

Climate studies also show that precipitation extremes — including floods and droughts — are likely to become more frequent and more intense in coming years as levels of greenhouse gases in the air continue to rise. For example, this study, published in the journal Science in 2004, found that much drier conditions may affect the Western US as a result of a warming climate, as has occurred in the distant past. However, studies have not yet tied specific drought events in the United States to manmade global warming.

As for tornadoes, I explored this topic in depth in previous stories for Climate Central, as well as at the Washington Post'sCapital Weather Gang blog. The gist is that studies show there are some links between La Niña years and major tornado outbreaks in particular, but there is also a lot of variability in this relationship. Computer modeling studies that simulate how a warming world may affect tornadoes show that atmospheric conditions may become more favorable for producing tornadoes, since a warmer ocean and atmosphere will provide more water vapor to fuel severe thunderstorms. However, wind shear — winds differing in speed and/or direction with height — may decrease as the planet warms, robbing severe thunderstorms of a key ingredient necessary for tornado formation.

So far, the research on tornadoes and climate change is far from conclusive, and observational records don't show any significant trends in tornado strength or frequency.

Climate Central research scientist Heidi Cullen explored the relationship between tornadoes and climate change on ABC's "World News Tonight" as well as "Nightline" last night.

What is clear, however, is that population growth has put more people in harm's way, and high death tolls can still occur despite accurate and timely warnings, particularly when tornadoes strike densely populated areas, as was so sadly demonstrated in Tuscaloosa and Joplin. The raw numbers are striking. According to the National Oceanic and Atmospheric Administration and media reports, 481 people are estimated to have died from tornadoes this year so far. The deadliest year in the official record (which begain in 1950) was in 1953, when 519 people were killed.

The bottom line is that climate scientists have long been warning that weather and climate extremes are likely to become more frequent and severe as the climate continues to warm. In many ways you can think of these recent events as "the new normal," but there are some limits to this concept. It's important to differentiate between the various types of extreme events, because each is influenced by many different factors in addition to climate change.

One of our key goals at Climate Central is to help you sort out fact from fiction in climate science. What are some of the climate change-related questions that you've been asking in response to these costly and deadly events?